The proposed project is concerned with biophysical mechanisms underlying the light-induced current fluctuations recorded from the ventral photoreceptor of Limulus under voltage-clamped conditions. These fluctuations are related to the "quantum bumps," which have been shown to be the responses to photons. The bump parameters, shape, size and rate, can be deduced from the fluctuations superimposed on the steady state response. We will characterize the fluctuations by the power spectral density -- a measure of the frequency contents of the fluctuations. From the power spectral density measured at different light intensities, we hope to deduce plausible biophysical mechanisms involved in light-induced conductance changes. Since calcium ions have been shown to have dramatic effects on the photoresponse, we plan to study the effects of calcium on the bump parameters in order that we may characterize the former in terms of the latter. Such a study should be valuable in gaining further insight into the biophysical mechanisms of phototransduction.